Off the top of my head:
File Number (from unit OR or last 3 of Svc No and (Pers))
SUBJECT (all caps, underlined)
Refs: A. .....
1. Body of text ......
2. Body of text ......
3. Body of text ......
Enclosures: # (if needed)
a. If you are unsure of the overall impression of the memo, ask your unit clerk to read it over, checking for format and the way the content is presented.
b. Only use official abbreviations, if you are not absolutely sure it is correct, write it out in full.
c. Writing in the third person, though often seen, is not required. Writing in the first person is acceptable.
d. Include copies of the reference documents if there is any chance the recipient may not have copies already.